I think what you say and what you do are two different things.
What you say:
Start off by letting him know that he isn’t alone. It is a struggle that all males deal with. However – it is an addiction like any other. And just like substance abuse eventually affects more than just the user – his pornography addiction is going to affect him as well as those around him. (Most notably – the girls he will date and the one he will eventually marry.)
Pornography is simply an investment in false reality and false intimacy. It is a mutilation of something that was created by God and intended to be something good.

If your son has a relationship with God – then I would encourage you to talk to him about it. I truly believe that the Holy Spirit moves in situations like these. My guess is that there is some sort of guilt associated with his actions.

What to do:
Get rid of the freakin’ internet. You are not the first parent who has dealt with this issue. In fact – you are already ahead of many parents just by being aware of the problem. What I tell many parents is to simply eliminate the temptation. Emails and online bill-paying are not as important as the mental health of your child.
If that is out of the question - scale down to one computer in the house and put it in your bedroom. I doubt he will be as interested in looking at pornography with you brushing your teeth two feet away.
Also...
Your son (every son…every person ) – should have an accountability partner or group. Someone in his life (preferably an older male) who is not afraid to ask your son the difficult questions. And, someone whom your son is willing to tell EVERYTHING to. I know a lot of parents want to be this person for their children but it just doesn’t work that way. I mentor 5 high school boys – and ALL of them have dealt with pornography at one point, but it is something that we were all as a group able to talk about openly and honestly.

Good Luck and please feel free to ask more questions. Like I said - you and your son are not alone in this.